The invention relates to a steel substrate structure which receives glass coatings and, in particular, it relates to a substrate steel plate compatible with glass or ceramic coating methods utilizing elevated temperature processing or firing cycles. The term "glassed steel" refers to a composite material with a substrate structure which is at least 3/16" in thickness and a glass or ceramic coating approximately 0.035 to 0.075" thick.
Glassed steel structures, which are generally constructed with shaped steel plates as substrates, are frequently used in process vessels for the chemical, petrochemical, pharmaceutical and food processing industries. Coatings prepared from silicate frits, with or without additives, have the greatest industrial usage. Variations in composition of silicate frits are virtually unlimited; frits range from alkali-alumina borosilicate glasses, which are relatively soft (low-melting) and highly fluxed, to barium crown glasses. Crystallized ceramic coatings wherein the crystallization of the glass is controlled by formulation and heat treatment and by the presence of nucleating agents which are added to the glass during melting are also known.
Chemical compatibility of the glass coating with the substrate steel is important and further the coating must also be physically compatible with the underlying metal structure so that undesirable mechanical stresses are not induced in either material during thermal processing.
The best combination of glass (or ceramic) and metal is obtained when the glass has a slightly lower coefficient of thermal expansion than the metal substrate. This "controlled mismatch" results in the glass coating being in a state of compression after cooling which is important since these coatings are much stronger in compression than in tension.
Coating processes include heat treatment to promote bonding and sealing. While cooling from such heat treatment, it is important than an unacceptable mismatch not occur in the expansion of the substrate relative to that which the coating will endure during phase transformation of the substrate. Therefore, there should be no lower temperature transformation products such as bainite or martensite formed in the steel substrate. If these products occur, cracking or crazing of the glass coating is likely due to severe expansion of the steel substrate at temperatures where the glass coating is not sufficiently plastic to endure the strain.
Coatings are usually applied by wet spray, hot dusting, or dipping methods. Before application by any method, the substrate metal surface should be clean and rough. Abrasive blasting is normally utilized to provide a satisfactory surface condition. Other surface preparation steps may be involved. Finally, in the application, the substrate metal receives a number of coatings with firing after each coating at temperatures up to about 1700.degree. F. Thus, for a quality product, it is essential that the steel substrate have physical characteristics whereby: (1) it will undergo the various steps utilized in the glassing process; (2) it does not promote cracking or crazing of the glass coating; (3) it provides an adequate structural support for the glass coating considering the ultimate use of the product; and (4) it does not deteriorate with the various heat treatments and other steps necessary for applying the glass to the steel substrate.
It is known to provide steel substrates for glassing processes which are relatively free of carbon, that is a low carbon steel plate or a steel plate in which the carbon has been stabilized by the addition of titanium such as ASTM A562 steel. ASME Code-approved carbon steels which are currently being used for glassing applications are known by the following ASME numbers: SA285 Gr. B, SA515 Gr.65, and SA516 Gr. 70. However, substrate structures composed of these steels are somewhat limited as to their ability to be successfully glassed without defects in the coating. Therefore, a need has existed for a better substrate steel which is more "glassable" than existing steels for both currently used and advanced coatings. In addition, improved mechanical properties of plate form the steel are desired, particularly notch toughness in the range of -20 to -50.degree. F., in both the longitudinal and transverse directions, such quality to exist after a plurality of heat treating cycles necessary to fuse the glass and bond it to the metal.
The following publications will aid in an understanding of the background of the invention:
Smith, Robert E., "Ceramic-Metal Composites," Chemical Engineering, May 10, 1965, pp. 194-200.
Dormer, George J., Norton, George R., and Payne, Burton S., "Glassing Characteristics of Low-Alloy Steels," Journal of the American Ceramic Society, Vol. 44, No. 8, Aug. 1961 pp. 375-381.
Payne, Jr., B.S., "Nucerite--A New Composite," Chemical Engineering Progress, Vol. 64, No. 2, February, 1968, pp. 40-43.